It is known that children suffering from muscular dystrophy are most often affected by a very severe form, Duchenne's muscular dystrophy (DMD). Due to the fact that the muscles of the trunk are attacked, these children almost all develop scoliosis.
The main characteristic of DMD lies, moreover, in the constant presence of a progressive respiratory insufficiency, the principal factor in a poor life prognosis. The current therapeutic advances in this field permit survival for a great many years. This explains the extreme importance of providing improved comfort in the seated position; the preservation of the serious deformations of the spine, sagittal (kyphosis) or frontal (scoliosis), is thus imperative.
As a result of this progressive respiratory insufficiency, as well as cardiac problems at a relatively advanced stage of DMD, the anaesthetic risks become much greater starting from an average age of about 13-14 years, a period in which the worsening of the scoliosis becomes very marked. It is for this reason that it has been proposed, since 1982, to treat this spinal deformation when the first signs appear indicating with certainty the future presence of scoliosis. Thus, all children with DMD can, without exception, benefit from surgical treatment of the spine: the more severe the DMD, the earlier the intervention will be.
The aim is therefore to fix a spine in its growth phase, seeking a preventive effect rather than a corrective effect, with the need to take into consideration a physiological position of the pelvis, the condition for maintaining a satisfactory equilibrium of the trunk in a permanent manner. The period of operability is situated, on average, between 10 and 13 years of age.
Moreover, this instrumentation must therefore be designed in such a way as to fulfil a double function: maintain spinal stability in the frontal plane and ensure a certain anteroposterior mobility, or mobility in the sagittal plane. This degree of spinal mobility promotes a balancing of the trunk, which improves the functional possibilities of the upper limb. Indeed, obtaining too rigid a spine constitutes an additional functional handicap, when the function of the upper limbs is greatly impaired.
The instrumentation developed by LUQUE is thus known, which extends over the lumbar and dorsal spine and consists of two L-shaped rods, connected at each level to the posterior arches of the vertebrae by metal wires placed around the laminae. The stresses are at their maximum on the steel wires at the ends of the rod effecting the convexity, and on the steel wires at the middle of the rod effecting the concavity. This instrumentation makes it possible in theory to restore a good curvature in the sagittal plane (lumbar lordosis and dorsal kyphosis), but the experience gained reveals a great many long-term problems with this instrumentation.
Complications have been noted for example, such as migration or breaking of the rod in a relatively large number of cases, after periods of six months to three years.
The instrumentation developed by COTREL-DUBOUSSET is also known, the lumbar part of which is firmly fixed, while the thoracic or dorsal part follows the segmental vertebral fixation of Luque, without arthrodesis, in order to permit the growth of this part of the spine. A relatively large number of material defects have been observed with this instrumentation, in particular fatigue breaks in rods stressed by considerable mobility of the truck of the patient. Moreover, with these known types of instrumentation, the growth of the spine provokes a crankshaft effect in the thoracic part, which means that it is not possible to control all the pathological curvatures at this level.
The principal causes of breaking found in conventional rods are the following: fatigue failure if there is no posterior arthrodesis and if the equipment is too rigid or if the stresses are not well distributed; diameter and resistance of the rod too low; insufficient restoration, or no restoration, of lumbar lordoses and dorsal kyphoses in the sagittal plane.
This latter point is very important, since poor restoration of the lumbar lordosis compromises the entire equilibrium of the superjacent spine. A substantial residual kyphotic curvature in fact subjects the rods to considerable stresses and clearly promotes the subsequent breaking. The breaks suffered by the rod are more frequent at the level of the dorsolumbar hinge. In the flexion/extension movement, the lumbar spine has a greater amplitude than does the dorsal spine, which is controlled more by the costal grill.